The present invention relates to an apparatus arranged and adapted to separate ions temporally according to a physico-chemical property such as ion mobility and a method of separating ions temporally according to a physico-chemical property such as ion mobility.
In many analyses involving separation, particularly for a targeted analysis where the mass to charge ratio and/or drift time of analytes of interest is known, the maximum ion mobility drift time of the targeted analyte ions at a given chromatographic retention time is generally known.
In order to improve the duty cycle of an ion mobility separator it is known to accumulate ions in an ion trap upstream of an ion mobility separator prior to releasing the ions into the ion mobility separator and allowing the ions to separate temporally according to their ion mobility. It is advantageous to operate with the fastest ion mobility separation cycle time practical.
Operating at the fastest practical ion mobility separation cycle time reduces the amount of charge which is accumulated in the upstream ion trap prior to ion mobility separation. This limits ion losses or and limits reduction in the performance of the ion mobility separation device due to space charge effects prior to separation.
In addition, relatively fast ion mobility separation cycle times (and hence short accumulation times) advantageously limits the amount of charge for a given analyte within the ion mobility separation device during separation. It is known that space charge effects within ion mobility separation devices can result in distortion of ion mobility separation peak widths and can also result in shifts in measured ion mobility separation drift times.
Furthermore, reducing the number of ions exiting the ion mobility separation device per ion mobility separation cycle reduces the requirement for high dynamic range ion detectors and recording electronics downstream of the ion mobility separation device.
It is therefore advantageous to analyse an ion population eluting from a chromatographic device (such as a liquid chromatography separation device) using a relatively large number of sequential fast ion mobility separation cycles with short accumulation times rather than analysing the ions eluting from the chromatographic device using only a relatively small number of ion mobility separation cycles having longer accumulation times. This maximises the dynamic range over which the entire ion population can be recorded.
However, in many cases, ions from matrix species or other non targeted analyte species which have a relatively low ion mobility and hence will have a longer drift time than the drift time of the analyte ions of interest will still be present in the ion mobility separation device after analyte ions of interest have exited the ion mobility separation device. Accordingly, if a second population of ions is then introduced into the ion mobility separation device before ions which are not of interest and which have relatively low ion mobilities have exited the ion mobility separation device then the ions which are not of interest and which have relatively low ion mobilities will still be present in the ion mobility separation device when the second population of ions is introduced. Accordingly, the undesired ions having relatively low ion mobilities will undesirably appear in the second ion mobility separation cycle and will appear to have relatively short ion mobility separation drift times. Accordingly, the undesired ions which remain in the ion mobility separation device after analyte ions of interest have exited the ion mobility separation device will cause aliasing or wrap around effects. The resulting aliasing or wrap around effects may result in interferences and/or misassignment of ion mobility or calculated collision cross section (“CCS”) values.
The problem of aliasing or wrap around effects is a particularly serious problem in a High Definition MSe (“HDMSe”) type experiment wherein fragment ions resulting from fragmenting parent or precursor ions which emerge from an ion mobility separation device may interfere in mass to charge ratio and ion mobility.
US 2010/0032561 (Micromass) discloses an ion tunnel device which may be operated in a first mode wherein ions are separated temporally according to their ion mobility and in a second mode wherein ions are separated temporally according to their mass to charge ratio.
US 2010/108879 (Micromass) discloses a mass spectrometer comprising an ion mobility separation device. In one arrangement, once ions with a desired charge state have exited the ion mobility separation device, the AC or RF voltage applied to the ion mobility separation device may be removed so that any undesired ions still in the device are no longer radially confined and hence are allowed to disperse.
WO 02/071439 (Tanner) discloses a mass spectrometer comprising a processing section such as a collision cell. An axial field and a DC voltage flush pulse may be applied to the collision cell in order to affect the charge distribution within the collision cell.
It is desired to provide an improved mass spectrometer and an improved method of mass spectrometry.